Methods for carburizing workpieces made of steel are used to harden the surface of the workpieces. In order to do this, in workpieces made of steel the surface layer of a low carbon steel is enriched before hardening of the workpiece takes place.
The carburizing takes place, for example, as low pressure carburizing. To this end, the workpieces are inserted into a vacuum furnace having a process chamber, to perform the carburizing. The process chamber has a process gas, that gives off carbon, flowing through it, in order to enrich the surface region of the workpieces with carbon.
In low pressure carburizing having thermal decomposition of the process gas, generally of hydrocarbons, in the process chamber, the process gases are generally injected during the individual process steps in a pulsating manner, the process chamber being alternatingly evacuated and/or exposed to flushing with nitrogen.
Such a method is discussed in DE-A 102 09 382, for example. In this case, the volume flow of the carburizing medium is regulated and varied in a controlled manner, during a pressure pulse, in such a way that uniform carburizing of a workpiece, that is to be carburized, is achieved.
However, in some workpieces it may be desirable that the surface of the workpiece have different hardnesses. This is the case, for instance, as discussed in DE-A 35 36 452, in the case of fuel injector nozzles for internal combustion engines. In that case it is required that the regions of the valve seat demonstrate a lower hardness than the outer region. In that case, the different hardness is achieved by first removing a part of the layer of the carburized and nitrided or nitrocarburized outer wall region, after the carburizing and after the nitriding or nitrocarburizing, before hardening the workpiece.
A disadvantage of this method is that the workpiece has to be manufactured oversized, so that after the carburizing or nitriding or nitrocarburizing one is still able to remove material.